Results for objective 1, research question 1 and Publication 2

The study performed in P2 was conducted as a practical application of CO2PE! UPLCI methodology and to compare metal L-PBF and CNC machining based on design

flexibility and effect on combined specific energy consumption (SEC) when multiple components are manufactured. Manufacturing scenarios were used to investigate evidence of how both methods contributed to sustainability. P2 aimed to conduct a detailed LCI study and to compare the sustainability aspects based on SEC, the material consumption and created scrap metal rate using PBF and CNC machining. Metal L-PBF samples were manufactured on a modified research machine representing EOSINT M-series and the machining was performed on a PUMA 2500Y CNC lathe machining centre. Figure 4.3 shows the CAD models and manufactured samples. The internal shape of samples B and C were similar in geometry. Sample B was designed as solid-walled and sample C as hollow-walled. The sample geometries used in this thesis were simple compared to the degree of complexity that can be achieved with L-PBF. The simple geometries were used in this thesis for manageable and equal comparison of the energy consumption, raw material usage and generated waste. In practice, however, the increasing complexity of components bridges the costs gap between AM and CNC machining. Detailed CAD models, manufactured components and manufacturing machines are shown in Appendix M2.

Figure 4.3: Representation of (a) cross-sectional views of the CAD models and (b) views of the manufactured components of samples A, B and C as designed in Publication 2.

Figure 4.3 shows the variance in the component designs of samples. The samples A, Bi and C were manufactured with L-PBF and Bii with CNC machining. L-PBF successfully built the hollow walled, chamfered outside geometry and sharp corners of Sample A as well the Samples Bi and C. Sample B was the only option to be manufactured with CNC machining as shown in Figure 4.3 Bii. The feasibility of machining samples A and C was assessed using simulation analysis which proved to be challenging, even if it had been

possible. The result proved that using CNC machining would otherwise require extensive design modifications even if CNC machining could manufacture samples A and C.

The time taken to manufacture the sample Bi with L-PBF was much and this increased the total consumed energy during the building. This was because the design was not optimised enough to take full advantage of the process which would have reduced the build time which directly affects the energy consumption. The surface finish of L-PBF parts (Bi) was course and this is one of the main shortfalls of L-PBF. Additional post-processing is often required to sublime metal L-PBF components to the desired smoothness in terms of appearance if it would be required in the use phase. Additional post-processes to smoothen the as-built parts attract extra energy, time consumption and costs. The ability to build the optimised designs, as samples A and C with L-PBF showed the contribution to enhancing material efficiency without affecting the form and design objective. Experimental testing would be necessary in order to ascertain the level to which the different samples could perform mechanically against predefined functional requirements.

The second part of P2 demonstrated the results of the LCI study of L-PBF and CNC machining. The results shown in Figure 4.4 are based on the identified machine levels, system boundaries and parameters in P1.

Figure 4.4: Summary of the results of the LCI study in (a) L-PBF and (b) CNC machining from Publication 2.

Figure 4.4 shows the measured ECUs, input and outputs of the comparable manufacturing methods. As can be seen from Figure 4.4, outputs such as heat, gases and noise were not measured in P2 as they were defined within the study boundary. The results of P2 contributed to answering R1 with the numerical values of the LCI study based on the identified system boundaries, machine levels and parameters for L-PBF and CNC machining. Machining three samples of Bii with CNC machining required three times the energy (10.56 MJ); to make one part (3.52 MJ); the same applied to the raw material (550 g) as can be seen in Figure 4.4b. The opposite was the case for metal L-PBF as the combined build on the build platform translated into better material and energy efficiency.

Energy efficiency is shown with a reduced combined energy consumption as the number of parts increased. L-PBF offered a means of achieving better-optimised part geometry with improved material efficiency. The material was saved using the reduced amount of start-up powder that would be needed for separate builds. This also translated into better indirect energy efficiency in terms of the saving in the embodied energy used to produce the raw material.

The results in P2 showed that specific energy consumption (SEC) for making samples was high in metal L-PBF. A comparable analysis of energy consumed for making one of sample B was almost ten times in L-PBF (39.9 MJ) compared to the quantity consumed to manufacture one sample B using CNC machining (3.52 MJ). It is worth stating that the experimental study was carried out using a modified version of the L-PBF machine.

Therefore, the energy consumption does not represent the industry perspective. The high

energy consumption in metal L-PBF however agrees with the similar observation of Liu et al. (2018); Sharif Ullah et al., (2015).

The result of the LCI study included a scenario-based analysis of specific energy consumption for metal L-PBF and CNC machining for making only sample B. Estimation of the effect of the batch size on specific energy consumption is based on computer data.

The scenario for metal L-PBF was based on simultaneous manufacturing of an increased number of parts to efficiently utilise the build platform. Figure 4.5 shows the result of the comparison of specific energy consumption for metal L-PBF and CNC machining.

Figure 4.5: Overview of the effect of batch size on specific energy consumption for manufacturing sample B with L-PBF and CNC machining.

The SEC per part for metal L-PBF was high, whereas CNC machining energy was low and constant. The combined value of SEC in metal L-PBF offered a decrease in SEC (dividing SEC per number of components on build platform) whereas combined CNC machining values increased with multiple part manufacturing. The combined SEC in CNC machining was estimated as a product of the individual SEC per one sample of Bii and the number of components that were manufactured. Figure 4.5 showed a reduction in SEC for metal L-PBF with the combined build of multiple Bi parts. This was because the amount of electrical power consumed by, for example, heating units, spreading powder, moving the build platform, laser chiller unit, servo motors and lightning system remained constant, irrespective of the number of manufactured parts. The scenario-based case showed that metal L-PBF could reduce raw material and energy consumption by combined manufacturing. Machining of the sample Bii on the other hand required the same amount of electrical power to separately machine multiple numbers of sample Bii.

The trend of SEC per estimated number of components shown in Figure 4.5 indicated that increasing the number of parts improves energy efficiency in metal L-PBF. Increasing the number of components in CNC machining showed a constant specific energy consumption per part, whereas SEC values decreased with multiple parts in L-PBF. The high energy consumption in metal AM can therefore be controlled by optimising the process parameters and build platform utilisation. The varying energy consumption in metal L-PBF confirms the observation of Sharif Ullah et al. (2015).

Some aspects of sustainability were identified to be better in metal L-PBF than in CNC machining based on the results of the scenario-based case in P2. Metal L-PBF offers optimised lightweight components and flexibility of an integrated design manufacturing.

Metal L-PBF enables better material utilisation, the absence of cutting tools (see Figure 4.4) reduced manufacturing steps, the extensive need for process fluid and reduces waste.

Waste is reduced in metal AM/L-PBF compared to CNC machining through better material usage, reduced scrap and process fluids. The assumption and empirical results of better material efficiency in metal AM/L-PBF, as can be seen from Figure 4.4, correspond to Najmon et al. (2019); Salmi et al. (2016); Serres et al. (2011).

The results of the P2 does not present the benefits such as post-consumer use value in this study. It is however presumed that EOL metal components are suitable for repairing, remanufacturing, recycling and recovering after their useful life. The LCI study in the P2 result does not also include empirical measurement of created emissions.